Generally, vehicles such as automobiles are equipped with an airbag unit that expands and unfolds an airbag for absorbing impact on an occupant in an emergency case, for example collision and sudden deceleration. In addition, various types of airbag units have thus far been developed such as a driver airbag unit installed inside the steering wheel, a passenger airbag unit installed inside the instrument panel, a side airbag unit installed inside a lateral portion of the vehicle or inside the seat, a curtain airbag unit installed inside an upper portion of a door, a knee airbag unit for protecting the knees of an occupant, and a pedestrian airbag unit installed under the engine hood.
The airbag units generally include an airbag which is normally folded but expanded and unfolded in an emergency case, an inflator that supplies gas to the airbag, and a retainer that holds the airbag and the inflator. Further, airbags have been proposed that include a vent hole that allows the gas in the airbag to be discharged to outside and are configured to adjust the internal pressure of the airbag by controlling the opening and closing of the vent hole, for example as disclosed in Patent Literature (PTL) 1 to 3.
The airbag unit according to PTL 1 includes a variable vent hole that allows or restricts the flow of the gas in and outside of the airbag depending on the unfolding state of the airbag, and a flowing status modification unit that modifies the gas flow through the variable vent hole. The flowing status modification unit is configured, while the airbag is unfolded in stages in a plurality of directions, to allow the gas to flow in an initial expansion stage of the airbag and then selectively allow and restrict the gas flow after the initial expansion stage depending on the unfolding state of the airbag.
The airbag unit according to PTL 2 includes an open/close control unit to which a strap is removably attached, and a flap member connected to the distal end portion of the strap and disposed along the periphery of the vent hole so as to cover the vent hole. The flap member presses the periphery of the vent hole so as to close the vent hole while connected to the open/close control unit while the airbag is expanding, and opens the vent hole upon being disconnected from the open/close control unit while the airbag is expanding.
The airbag unit according to PTL 3 includes a first strap connected to a peripheral portion of the vent hole and a lower portion of an occupant-side base cloth of the airbag, and a second strap connected to an edge portion of the vent hole and a rear portion of the occupant-side base cloth of the airbag. The first strap is disposed so as to close the vent hole when the airbag is expanded and unfolded, and the second strap serves to pull the first strap together with the edge portion of the vent hole so as to open the vent hole, when the occupant is caught by the airbag.